1. Field of the Invention
The present invention concerns a method to correct the field strength of radio-frequency pulses, which in a magnetic resonance MR examination are emitted by an antenna of a magnetic resonance examination apparatus.
2. Description of the Prior Art
Magnetic resonance tomography is an increasingly employed technique to acquire images of the inside of the body of a living examination subject. In order to acquire an image with this modality, the body or a body part of the examination subject must first be exposed to as homogenous a static basic magnetic field as possible, which is generated by a basic field magnet of the magnetic resonance measurement device. During the data acquisition for the magnetic resonance Images, rapidly switched gradient fields for spatial coding, which are generated by gradient coils, are superimposed on this basic magnetic field. Radio-frequency pulses of a defined field strength are then emitted into the examination subject with a radio-frequency antenna. The magnetic flux density of these radio-frequency pulses is typically designated as B1, the pulsed radio-frequency field is generally also called a B1 field for short. By means of the radio-frequency pulses, in the examination subject magnetic resonance signals are excited which are acquired by a radio-frequency reception antenna. The reception antenna can either be the same antenna with which the radio-frequency pulses are emitted or a separate reception antenna. The magnetic resonance images of the examination subject are then generated on the basis of the received magnetic resonance signals. A small physical volume, known as a “voxel,” is associated with each image point in the magnetic resonance image. Each brightness or intensity value of the image points is linked with the signal amplitude of the magnetic resonance signal received from the voxel. The strength of the magnetic resonance signal is also dependent on, among other things, the strength of the emitted B1 field. Oscillations in the field strength of the excited B1 field thus lead to unintentional changes in the received magnetic resonance signal that can falsify the measurement result.
Typically, multiple transmission antennas are employed in magnetic resonance imaging as resonant antennas. Such antennas are differently energized by different loads, which, given constant feed power, leads to radio-frequency field strengths of different amplitudes. The load affecting the antenna is substantially dependent on, among other thing, the position of the examination subject in relationship to the antenna. Consequently, a new positioning of the patient between two magnetic resonance measurements within an examination, or an accidental movement of the patient, inevitably lead to a change of the antenna load and thus, given the same feed power, to a change of the B1 field. For this reason, a new adjustment of the transmitting power is typically implemented with each new positioning of a patient, in order to again set the B1 field to the correct value. Such an adjustment measurement is relatively complicated. As a rule, for this purpose the transmitting power is modified until, given a predetermined duration of the transmission pulse, by the influence of a radio-frequency pulse a specific, precisely measurable flip angle is set between the nuclear magnetization and the homogeneous magnetic basic field. Given the known flip angle and known pulse duration, the actual B1 field that exists given the appertaining transmitting power is determined. As a rule, the adjustment (calibration) ensues at a flip angle a of 180°, meaning at a location in which the nuclear magnetization is opposite to the static magnetic field, since in this case no magnetic field component exists transverse to the magnetic basic field. This transverse magnetization can be easily directly verified by the signal (free induction decay, FID) induced in the radio-frequency coil after the end of the exciting radio-frequency pulse. Therefore, to adjust the B1 field, only the transmitting power must be varied, until the received FID signal is equal to zero.
A problem with this method is that, given multiple examinations, in particular given whole-body scans (such as, for example, in examinations in which blood vessels should be shown from the center of the body to the legs, using contrast), a faster measurement process is imperative. For such measurements to be implemented quickly, complicated transmitting power adjustments in the case of a new positioning of the patient can not be implemented for reasons of time. The adjustments are therefore often foregone at the cost of image quality.